My overall objectives are to discover, test and apply correct theoretical principles governing molecular electronic structure. In this specific project, the aim is to elucidate the qualitative and quantitative factors essential for understanding changes of energy accompanying changes of molecular conformation, including molecular vibrations far from equilibrium and molecular internal rotations. Employed are calculations solving the Schorodinger equation, formal analysis based on the equation, and experimental spectroscopic data. Development of basic concepts is emphasized, with a special focus on the role of electron density. Simple and practical schemes are being sought for actual prediction of molecular properties, especially potential energy surfaces.